Yahoo's best tools and features include the following:
Yahoo Site Explorer - This tool shows a large portion of the links that Yahoo is aware of (but not all of them). It does so for any website, making this a neat competitive link intelligence tool. You'll hear many SEOs say that Yahoo links don't matter in Google, but Google does not show the inbound link profile for any site, so this is the only tool that we have. Features include "Inlinks to all pages," "inlinks to URL," "inlinks except this domain." You can also use Majestic SEO and Linkscape for link data.
Yahoo Keyword Tool - To get access to this tool, you need to have a Yahoo Search Marketing account. If you don't want to pay them $50 and open an account, just follow the registration steps until you get to the keyword tool; you won't need to actually finish registration to use the tool.
Advanced Search - Use the advanced search tool to look for .gov, .edu and other quality links. You'll actually find check boxes designed for that purpose.
Yahoo Universal Search - Yahoo followed Google's lead and introduced Universal Search.
Yahoo Maps - Yahoo owns a map service called Yahoo Maps, and features business listings for local search results like "new York pizza" or "Montreal limo." The results are similar to Google, but are limited to three listings.
Videos and Images - Yahoo also blends videos and images in search results. It also experimented with featuring video ads for some brand searches like Honda. At one point there was a campaign on TV that went something like "search for Honda on Yahoo" and the results gave a Honda-sponsored listing with video and detailed descriptions of a car.
Celebrity searches yield a lot of images. Yahoo also owns Flikr, a popular image hosting website, so they bias their results towards Flikr.
News - Yahoo News works in a similar way to Google News, but with more of a bias towards their own content. Search for politicians, celebrities and any keywords that have a lot of news and you'll see blended news results.
Conclusion
Aggressive optimization for Yahoo can actually hurt Google rankings. Yahoo still holds considerable market share (~20%), so it makes sense to optimize for some of their traffic, as long as it doesn't come at the cost of Google rankings.
The fastest way to be featured in Yahoo search results is through the Yahoo Search Submit program.
Yahoo Optimization 
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